1. Field of the Invention
The present invention relates to an optical waveguide having photosensitivity in the core thereof, and more specifically to a production device for a grating-type optical component and a production method for a grating-type optical component, the properties of which are adjusted by changing the refractive index thereof using ultraviolet light (hereinafter “UV”) and to an optical component made using the production device for a grating-type optical component or production method for a grating-type optical component.
2. Description of the Related Art
Quartz is a material having excellent optical transmission qualities and is therefore used in a variety of applications such as optical lenses or waveguides for optical transmission or the like. The material of an optical fiber as an optical communications line is silica based, when producing functional optical components such as an optical wavelength selection filter, an optical splitter, a spectral separator or attenuator or the like in that line, there are merits, in terms of compatibility with optical fiber (the refractive index, core diameter, and fusion point when making a fusion connection), to produce such functional components of quartz.
In an optical waveguide such as a planar light wave circuit (PLC) or an optical fiber including photosensitive material such as Ge, phosphorus or boron added in the core, a grating is formed by irradiating UV light of an appropriate wavelength into the optical waveguide from the side thereof so as to alter the refractive index inside the core periodically, in a longitudinal direction; such gratings comprise optical components used as an above-mentioned wavelength selection filter. As shown in FIG. 1A, in accordance with the desired objective, the refractive index is varied at a determined periodicity following in a longitudinal direction of the optical waveguide 100. Further, besides a gradual, successive alteration, this alteration may be of an irregular or discontinuous periodicity.
The grating 102 formed in the core 101 of the optical fiber 100 is called an optical fiber grating. Normally, the optical fiber grating is classified by the period of refractive index variation. One is long period grating whose period of refractive index variation is above 100 μm, and the other is fiber Bragg grating (hereafter “FBG”) whose period of refractive index variation is below a few micro meters. These are important optical components in the field of optical transmission.
In the description following, optical fiber refers to optical waveguides. In the same manner, FBG in the description refers to a grating formed inside an optical waveguide.
When the period of refractive index variation of an FBG formed in a core is determined as Λ, wavelength λ of light reflected at the FBG satisfying the expressionλ=2·neff·Λ0  (1).
Here, neff is the effective refractive index of the FBG and neff is nearly equal to 1.46 at the silica-based core. As an example, if the above expression (1) is applied to wavelength λ=1550 nm used in public (commercial) optical transmission networks, then FBG pitch Λ0≈500 nm=0.5 μm is obtained.
A conventional method for producing an FBG will now be described. Referring to FIG. 2A, firstly optical fiber 127 is disposed inside pressurized container 111. A kilometer or from several hundred to tens of meters of reeled optical fiber covered with protective coating, optical fiber covered with protective coating cut into several meter lengths, or optical fiber 127 as shown in the drawing cut into several meter lengths having a part of the covered protective coating material part 129 removed to expose the inner part are all suitable for use as the optical fiber 127.
Next, in a condition loaded with hydrogen (H2) or deuterium (D2) and in a pressurized condition (e.g.: 10 MPa-30 MPa), high-pressure hydrogen 113 or deuterium is diffused through the cladding 125 of the optical fiber 127 reaching the core 123. This process is known as hydrogen diffusion treatment.
The object of the above hydrogen diffusion treatment is that if hydrogen or deuterium are diffused into the core 123 of the optical fiber 127 then, as will be described subsequently, the photosensitivity of the core 123 can be increased when an interfering UV laser beam is radiated to the core 123. In other words, it is known that when imprinting an FBG, defusing hydrogen or deuterium in a core, here core 123, raises the speed of the increase of the refractive index approximately fiftyfold in comparison to a core that has not been diffused with hydrogen or deuterium. It is well known that in such a condition, raising the temperature inside the pressurized container above room temperature raises the speed of this diffusion.
When, in this hydrogen diffusion treatment, the optical fiber 127 has been diffused with hydrogen or deuterium, the covering material part 129 must be removed to radiate UV laser rays therein. This is because the covering material part 129, of resin, diffused with the hydrogen, absorbs UV laser light thereby preventing the rays from reaching the core.
Next, as highly interfering UV laser light 171 is radiated through a phase mask 173 having a specific periodicity, a fringe pattern of the interference arises in the hydrogen diffused optical fiber core 123; the density of energy being higher, and thereby raising the refractive index, in the bright portions of this UV pattern. Usually, interference of diffracted light of first order through the phase mask is used, the resulting interference fringe being half the period of the phase mask such that the period of the FBG is half the period of the phase mask. An FBG (hydrogen diffused) 121 having an uniform period can be formed in this way. The process itself is known as UV exposure processing.
As shown in FIG. 2C, the optical fiber with imprinted FBG is then placed in an oven 151 for a determined period of time (e.g. 12 hours) in a heated condition (120° C.) so that the hydrogen 153 or deuterium diffused into the optical fiber 127 is released to the outside. This process is called the hydrogen removal process. The optical fiber 117 shown in FIG. 2C is an optical fiber with hydrogen removed through the hydrogen removal process, and the optical fiber covering part 119 thereof is a cladding, the hydrogen in which has been removed in the same manner.
An optical fiber having a refractive index periodically distributed at a constant pitch Λ0 in the core thereof inside a cladding produced in this way, as shown in FIG. 1A, is called a uniform type FBG. In a uniform type FBG reflection occurs at multiple points in phase in relation to signal light of wavelength λi satisfying the above expression (1), among signal light propagating in the core. Appropriate applications can be found in FBG for stabilization of wavelengths of laser diodes (“LD”) or FBG for Add/Drop for adding or dropping light with specific wavelengths.
Where the pitch Λ of an FBG inside a core changes successively and gradually (e.g. Λ1-Λn), the FBG is said to be a chirped type FBG. This kind of FBG has broad bandwidth and is effective for multiple wavelengths. Appropriate applications can be found in FBG for compensating chromatic dispersion and FBG for equalizing gain after amplification by an optical amplifier.